U.S. patent application number 10/306224 was filed with the patent office on 2003-06-26 for eye-safe optical fibre transmitter unit.
This patent application is currently assigned to Agilent Technologies, Inc.. Invention is credited to Healy, David.
Application Number | 20030118283 10/306224 |
Document ID | / |
Family ID | 8182501 |
Filed Date | 2003-06-26 |
United States Patent
Application |
20030118283 |
Kind Code |
A1 |
Healy, David |
June 26, 2003 |
Eye-safe optical fibre transmitter unit
Abstract
The present invention relates to an eye-safe optical fibre
transmitter unit (1), and to a method of producing such an unit.
The unit (1) comprises a laser diode (9), an optical port (24), an
optical fibre stub (18) with a fibre core (31) for carrying optical
radiation (51) emitted by the laser (9) to the optical port (24).
The core (31) is an index-guided core (31) surrounded by cladding
(29). Focussing optics (12) focussing laser optical radiation (36)
to a focus spot (50) on an entrance face (17) of the fibre stub
(18) in order to increase the coupling efficiency of optical
radiation (36,51) into the fibre core (31) and to decrease the
coupling efficiency of optical radiation (36,52) into the
surrounding cladding (29). The coupling efficiency into the core
(31) is a maximum at a particular orientation of the entrance face
(17) with respect to a focus axis (35) when the focus spot (50) is
on the entrance face (17). However, the entrance face (17) is not
oriented at this particular orientation, but is angled and/or
rotated (53) away from said particular orientation in order to
reduce the efficiency of coupling said optical radiation
(36,51,52,) into both the fibre core (31) and cladding (29).
Inventors: |
Healy, David; (Suffolk,
GB) |
Correspondence
Address: |
Allan M. Lowe
c/o Lowe, Hauptman , Gilman & Berner
Suite 310
1700 Diagonal Road
Alexandria
VA
22314
US
|
Assignee: |
Agilent Technologies, Inc.
|
Family ID: |
8182501 |
Appl. No.: |
10/306224 |
Filed: |
November 29, 2002 |
Current U.S.
Class: |
385/33 ;
385/88 |
Current CPC
Class: |
G02B 6/421 20130101;
G02B 2006/4297 20130101; G02B 6/4206 20130101 |
Class at
Publication: |
385/33 ;
385/88 |
International
Class: |
G02B 006/32; G02B
006/42 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 30, 2001 |
EP |
01310047.4 |
Claims
1. An optical transmitter unit (1,201), comprising a laser diode
(9) for emitting optical radiation (32,36,51,251), an optical fibre
stub (18) having a fibre core (31) for carrying said optical
radiation (51,251) and being surrounded by a cladding (29), said
optical fibre stub being disposed in a ferrule (19), and focussing
optics (12) for focussing said optical radiation (36) from the
laser diode (9) into an entrance face (17,40) of the fibre stub
(18), characterised in that: a) the focussing optics (12) focus the
optical radiation (36) along a focus axis (35) to a focus spot (50)
on the entrance face (17,40) of the fibre stub (18), said entrance
face being at a particular orientation with respect to said focus
axis (35); and b) said ferrule (19) being rotatable in a manner
which alters said particular orientation of said entrance face
(17,40) with respect to said focus axis (35), thereby affecting the
efficiency of coupling said optical radiation (36,51,52,251,252)
into said fibre core (31) and cladding (29).
2. An optical transmitter unit (1,201), comprising a laser diode
(9) for emitting optical radiation (32,36,51,251), an optical port
(24) for connection to an optical fibre transmission link
(23,25,28), an optical fibre stub (18) with a fibre core (31) for
carrying said optical radiation (51,251) to the optical port (24),
said core (31) being an index-guided core (31) surrounded by
cladding (29), and focussing optics (12) for focussing said optical
radiation (36) from the laser diode (9) into an entrance face
(17,40) of the fibre stub (18), characterised in that: a) the
focussing optics (12) focus the optical radiation (36) along a
focus axis (35) to a focus spot (50) on the entrance face (17,40)
of the fibre stub (18) in order to increase the coupling efficiency
of optical radiation (36,51,251) into the fibre core (31) and to
decrease the coupling efficiency of optical radiation (36,52,252)
into the surrounding cladding (29), said coupling efficiency into
the core (31) being a maximum at a particular orientation of the
entrance face (17,40) with respect to the focus axis (35) when the
focus spot (50) is on the entrance face (17,40); and b) the
entrance face (17,40) is not oriented at said particular
orientation, but is angled and/or rotated (53,253) away from said
particular orientation in order to reduce the efficiency of
coupling said optical radiation (36,51,52,251,252) into the fibre
core (31) and cladding (29).
3. An optical transmitter unit (1,201) as claimed in any preceding
claim, in which the entrance face (17,40) is angled at a non-normal
angle (47,247) to said focus axis (35).
4. An optical transmitter unit (1,201) as claimed in any preceding
claim, in which the entrance face of the fibre stub (18) is a face
(40) of a linear polarising element (39) with a polarisation axis
(60) set at an angle (62) to a linear polarising direction (61) of
the focussed optical radiation (36) in order to reduce the
efficiency of coupling said optical radiation (36,251,252) into the
fibre core (31) and cladding (29).
5. An optical transmitter unit (1,201) as claimed in any preceding
claim, in which the optical coupling efficiency into the fibre core
(31) is reduced by at least 10% from the maximum coupling
efficiency at said particular orientation.
6. An optical transmitter unit (1,201) as claimed in any preceding
claim, in which the emitted radiance from the unit (1,201) with no
optical transmitter link (23,25,28) connected to the optical port
(24) conforms to the US Standard CDRH Class 1 and/or the European
Standard IEC 825.
7. An optical transmitter unit (1,201) as claimed in any preceding
claim, in which the fibre stub (18) is less than 10 mm long.
8. An optical transmitter unit (1,201) as claimed in any preceding
claim, in which the transmitter unit (1,201) includes a housing
(2,7), said housing encompassing the laser diode (9), focussing
optics (12) and entrance face (17,40) of the optical fibre stub
(18), wherein the housing (2,7) is divided into parts by the
focussing optics (12) including a first part (70) encompassing the
laser diode (9) and a second part (71) encompassing the entrance
face (17,40) of the optical fibre stub, wherein the division
between the first part (70) and the second part (71) of the housing
(2,7) helps to prevent uncoupled optical radiation (55) in the
second part (71) of the housing (2,7) from reaching the first part
(70) of the housing (2,7).
9. An optical transmitter unit as claimed in any preceding claim,
in which said focus spot is a beam waist (50).
10. A method of assembling an optical transmitter unit (1,201),
said transmitter unit comprising a laser diode (9) for emitting
optical radiation (32,36,51,251), an optical port (24) for
connection to an optical fibre transmission link (23,25,28), an
optical fibre stub (18) with a fibre core (31) for carrying said
optical radiation (51,251) to the optical port (24), said core (31)
being an index-guided core (31) surrounded by cladding (29), and
focussing optics (12) for focussing said optical radiation (36)
from the laser diode (9) into an entrance face (17,40) of the fibre
stub (18), characterised in that the method comprises the steps of:
i) using the focussing optics (12) to focus the optical radiation
(36) along a focus axis (35) to a focus spot (50) on the entrance
face (17,40) of the fibre stub (18) in order to increase the
coupling efficiency of optical radiation (36,51,251) into the fibre
core (31) and to decrease the coupling efficiency of optical
radiation (35,52,252) into the surrounding cladding (29); ii)
orienting the entrance face (17,40) with respect to the focus axis
(35) to a particular orientation in order to maximise said coupling
efficiency into the fibre core (31) when the focus spot (50) is on
the entrance face (17,40); and then iii) changing the orientation
of the entrance face (17,40) with respect to the focus axis (35)
away from said particular orientation by angling and/or rotating
the entrance face (17,40) with respect to the focus axis (35) in
order to reduce the efficiency of coupling said optical radiation
(36,51,251,52,252) into the fibre core (31) and cladding (29).
11. A method as claimed in claim 10, in which step iii) involves
adjusting the angle (47,247) of the entrance face (17,40) with
respect to the focus axis (35) to a non-normal angle when the
entrance face (17,40) is oriented to reduce the optical coupling
efficiency.
12. A method as claimed in claim 10 or claim 11, in which the
entrance face (40) of the fibre stub (18) is a face of a linear
polarising element (39), and step iii) involves rotating (253) the
entrance face (40) with respect to the focus axis (35) in order to
increase absorption of incident optical radiation (36) by the
polarising element (39) prior to transmission of said optical
radiation (51) to the optical port (24) by the fibre stub (18).
13. A method as claimed in any of claims 10 to 12, in which the
focussing optics (12) have an focus axis (35) that is offset to a
projection axis (33) of the optical radiation (32) from the laser
diode (9), and the optical fibre stub (18) has a transmission axis
(30) that is angled to the focus axis (35) for optical radiation
(36) focussed onto the entrance face (17,40) of the fibre stub
(18), said entrance face (17,40) therefore being oblique to the
transmission axis (30) of the optical fibre stub (18) in order to
maximise coupled optical radiation (36,51,251,52,252) into the
optical fibre stub (18) at an optimal orientation between the
entrance face (17,40) of the optical fibre stub (18) and said
focussed optical radiation (36), step iii) then consisting of the
step of rotating (53,253) the optical fibre stub (18) about the
transmission axis (30) whilst otherwise maintaining the orientation
between the optical fibre stub (18) and the focussing optics (12).
Description
[0001] The present invention relates to an eye-safe optical fibre
transmitter unit, and to a method of producing such an unit.
[0002] Optical devices such as laser transceiver units often have
an optical port for receiving and/or transmitting laser light
from/into optical fibres, for example as part of an optical
communications system. The optical fibre has at its end a connector
by which the fibre may be connected and disconnected to the
port.
[0003] In the case of an optical fibre laser transmitter unit, when
the connector is not connected to the port, laser radiation from
the port needs to be eye-safe. In many cases, optical
communications links operate at near-infrared wavelengths of 1.3
.mu.m and 1.5 .mu.m, which presents added risk because such
wavelengths are invisible. Applicable eye-safety standards for
infra-red laser diode transmitter units are the US Standard CDRH
Class 1 and the European Standard IEC 825.
[0004] Current laser safety guidelines require that the output
power density from an optical port of an optical transmitter unit
be limited to a level which is eye-safe when no fibre optic
connected is connected to the port. Optical coupling efficiencies
from a laser diode into an optical fibre are typically quite low,
for example of the order of about 1% to 25%. Even if the amount of
laser radiation transmitted by the fibre is eye-safe, the total
amount of optical radiation emitted by the laser diode may far
exceed the limit of eye-safety. It is therefore necessary either to
block unwanted light within the port, or to defocus stray light
emitted by the port when no optical connected is connected to the
port.
[0005] One solution to this problem is disclosed in patent document
U.S. Pat. No. 5,315,680, which describes an optical port having a
short length of optical fibre, called a "fibre stub" held securely
in alignment with a laser diode concealed within an optical
transmitter unit. Collimating optics are used to focus the laser
light into a single-mode core of the fibre. The fibre stub is
typically 5 mm to 6 mm long. Light which is not coupled into the
core entered the fibre optic cladding, and is dissipated by
multiple reflections and scattering with the core and which the
exterior surface of the cladding. Any laser radiation that exists
from the cladding in not collimated, and is essentially
"defocussed" so that the inherent brightness of such stray
radiation is greatly reduced.
[0006] In recent years there has been an increasing demand for
fibre optic communication links having a bandwidth in excess of 1
GHz, for example up to 10 GHz. One way in which a laser diode can
be made to operate at higher data rates is to drive the laser at a
higher power. It may be possible to reduce the amount of optical
power launched into the core by defocussing a laser beam focussed
on the input end of the fibre stub, that is, by axially offsetting
the laser beam waist with respect to the entrance face of the fibre
optic core. Such a technique may also be used to vary (i.e. reduce)
the amount of optical power in the core depending on product
specifications and the requirements of various applications. The
core diameter is, however, much smaller than that of the cladding,
and so more defocussed light will be launched into the cladding.
Thus, there will still be more total laser power launched into both
the core and the cladding of the fibre stub, to the point where
light emitted by the fibre core and/or stub is no longer
eye-safe.
[0007] Another problem with using the defocus technique is that the
amount of laser power entering the core then becomes more sensitive
to changes in the relative orientation along the light transmission
direction, of the fibre stub, the laser and any intervening
collimating optics. Such orientations can change owing to thermal
expansion of components forming the optical transmitter unit, or
because of ageing-induced creep of the materials and adhesives used
in the construction of the unit.
[0008] One way to reduce the laser power emitted at the end of the
fibre stub is to increase the length of the stub in order to
increase scattering and absorption over the length of the stub.
Cladding modes within a length of optical fibre between about 100
mm and 200 mm long will be substantially dissipated. This, however,
results in an increase in the size of the optical transmitter
module, which is undesirable.
[0009] Another solution is to incorporate an aperture at the end of
the stub, for example by means of an absorbing ring around the
outside of the fibre core. The aperture, however, must be formed in
close alignment with the core, which is of the order of about 10
.mu.m in diameter. This results in additional process steps, which
add cost and complexity to the optical transmitter unit.
[0010] It is an object of the present invention to provide a more
convenient eye-safe optical transmitter unit, and a method for
manufacturing such a unit.
[0011] Accordingly, the present invention provides an optical
transmitter unit, comprising a laser diode for emitting optical
radiation, an optical fibre stub having a fibre core for carrying
said optical radiation and being surrounded by a cladding, said
optical fibre stub being disposed in a ferrule, and focussing
optics for focussing said optical radiation from the laser diode
into an entrance face of the fibre stub, characterised in that:
[0012] a) the focussing optics focus the optical radiation along a
focus axis to a focus spot on the entrance face of the fibre stub,
said entrance face being at a particular orientation with respect
to said focus axis; and
[0013] b) said ferrule being rotatable in a manner which alters
said particular orientation of said entrance face with respect to
said focus axis, thereby affecting the efficiency of coupling said
optical radiation into said fibre core and cladding.
[0014] Accordingly, the invention also provides an optical
transmitter unit, comprising a laser diode for emitting optical
radiation, an optical port for connection to an optical fibre
transmission link, an optical fibre stub with a fibre core for
carrying said optical radiation to the optical port, said core
being an index-guided core surrounded by cladding, and focussing
optics for focussing said optical radiation from the laser diode
into an entrance face of the fibre stub, characterised in that:
[0015] a) the focussing optics focus the optical radiation along a
focus axis to a focus spot on the entrance face of the fibre stub
in order to increase the coupling efficiency of optical radiation
into the fibre core and to decrease the coupling efficiency of
optical radiation into the surrounding cladding, said coupling
efficiency into the core being a maximum at a particular
orientation of the entrance face with respect to the focus axis
when the focus spot is on the entrance face; and
[0016] b) the entrance face is not oriented at said particular
orientation, but is angled and/or rotated away from said particular
orientation in order to reduce the efficiency of coupling said
optical radiation into the fibre core and cladding.
[0017] Also according to the invention there is provided a method
of assembling an optical transmitter unit, said transmitter unit
comprising a laser diode for emitting optical radiation, an optical
port for connection to an optical fibre transmission link, an
optical fibre stub with a fibre core for carrying said optical
radiation to the optical port, said core being an index-guided core
surrounded by cladding, and focussing optics for focussing said
optical radiation from the laser diode into an entrance face of the
fibre stub, characterised in that the method comprises the steps
of:
[0018] i) using the focussing optics to focus the optical radiation
along a focus axis to a focus spot on the entrance face of the
fibre stub in order to increase the coupling efficiency of optical
radiation into the fibre core and to decrease the coupling
efficiency of optical radiation into the surrounding cladding;
[0019] ii) orienting the entrance face with respect to the focus
axis to a particular orientation in order to maximise said coupling
efficiency into the fibre core when the focus spot is on the
entrance face; and then
[0020] iii) changing the orientation of the entrance face with
respect to the focus axis away from said particular orientation by
angling and/or rotating the entrance face with respect to the focus
axis in order to reduce the efficiency of coupling said optical
radiation into the fibre core and cladding.
[0021] By reducing the efficiency of coupling optical radiation
into the fibre stub, the amount of optical radiation emitted by the
transceiver unit when no optical transmission link is connected to
the transmitter unit can be reduced, and so made eye-safe. At the
same time, by focussing the optical radiation onto the entrance
face of the fibre stub, the proportion of optical radiation in
launched into the index-guided core with respect to the cladding,
is increased, so increasing the useful proportion of optical
radiation transmitted by the optical transmitter unit, compared
with wasted optical radiation in the cladding.
[0022] One way in which the relative orientation of the entrance
face and the focus axis can be set in order to reduce the coupling
efficiency is to adjust and set the angle of the entrance face with
respect to the focus axis at a non-normal angle. Unwanted optical
radiation will then be reflected by the entrance face, rather than
admitted and transmitted within the fibre stub.
[0023] Apart from the optical port, the optical transceiver unit is
preferably light-tight so that such stray reflections within the
unit are ultimately absorbed.
[0024] The entrance face of the fibre stub may include a polarising
element, for example a polarising isolator for suppressing
back-reflection of the optical radiation into the laser diode. Such
an isolator may be adhered onto an input facet of the fibre stub
within the optical transceiver.
[0025] The polarising element, whether or not the element is also
an isolator, may also be set at an angle or orientation at which
said polarising element reduces the coupling efficiency. This can
be done by rotating the polariser about the focus axis to an
orientation at which the polariser admits a reduced amount of
optical radiation into the fibre stub. The polariser then absorbs
an increased amount of optical radiation prior to transmission of
said optical radiation to the optical port by the fibre stub.
[0026] In a preferred embodiment of the invention, the focussing
optics have an optical axis that is offset to the projection axis
of the optical radiation from the laser diode, and the optical
fibre stub has a transmission axis that is angled to a focus axis
for optical radiation focussed onto the entrance face of the fibre
stub. The entrance face is therefore oblique, that is, angled at a
non-right angle, to the transmission axis of the optical fibre stub
in order to maximise coupled optical radiation into the optical
fibre stub at an optimal orientation between the entrance face of
the optical fibre stub and said focussed optical radiation. The
optical radiation coupled into the fibre core is then reduced by
rotating the optical fibre about the transmission axis whilst
otherwise maintaining the orientation between the optical fibre
stub and the focussing optics.
[0027] By changing the angle and/or rotation as described above, it
is possible to vary the coupling efficiency over a range whereby
the amount of optical radiation coupled into the fibre core is
reduced by at least 10%, and up to 90%, from the maximum coupling
efficiency at said particular orientation.
[0028] The invention will now be described in further detail, and
by way of example only, in which:
[0029] FIG. 1 is a cross-section of an optical transmitter unit
according to the invention, having a laser diode and focussing
optics that launch optical radiation into a fibre stub in such a
way that the coupling efficiency of optical radiation into the
fibre core and cladding is reduced by angling an entrance face of
the fibre stub with respect to a focus axis;
[0030] FIGS. 2 to 4 are schematic drawings of a prior art optical
transmitter unit similar to that of FIG. 1, in which the optical
fibre stub is angled and rotated to maximise optical coupling
efficiency into the fibre, while at the same time coupling
efficiency into the fibre core is reduced by defocusing the laser
beam waist with respect to the fibre core;
[0031] FIGS. 5 and 6 are schematic drawings of the optical
transmitter unit of FIG. 1, showing how the coupling efficiency of
optical radiation into the fibre core is first maximised by first
aligning the fibre core with respect to the laser beam waist, and
then rotating the fibre to reduce this coupling efficiency; and
[0032] FIGS. 7 and 8 are schematic drawings of a second embodiment
of an optical transmitter unit according to the invention, showing
how use of a polarising filter can help in reducing the optical
coupling efficiency into the optical fibre stub.
[0033] FIG. 1 shows schematically an optical transmitter unit 1
according to the invention, not to scale, having a body 2 with a
multiply-stepped axial bore 3. At one end 4 of the bore 3 is
mounted a laser transmitter module 5 secured to the body 2, for
example by an organic adhesive (not shown).
[0034] The transmitter module 5 comprises a base plate 6 (also
referred to as a "header"), which together with a metal can 7 forms
a sealed enclosure 8 housing a laser diode 9 mounted on a heat sink
10. Secured to the can 7 is a mount 11 holding a spherical lens 12
which collimates optical radiation output from the laser diode
9.
[0035] Electrical connections to the laser diode are via contact
pins 13,14 which project from the base plate 6.
[0036] A collimating lens 12 forms an aperture in the laser module
5 from which light is focused through a transparent window 15
towards an entrance face 17 of an optical fibre stub 18 securely
held within a cylindrical ceramic ferrule 19. The optical fibre
stub 18 includes an index-guided fibre having a single-mode core 31
surrounded by cladding 29.
[0037] The body 2 and can 7 together form a housing 2,7 that is
divided into parts by the focussing optics 12, including a first
part 70 encompassing the laser diode 9 and a second part 71
encompassing the entrance face 17 of the optical fibre stub 18. The
division between the first part 71 and the second part 72 of the
housing 2,7 helps to prevent uncoupled optical radiation in the
second part 71 of the housing from reaching the first part 70 of
the housing.
[0038] The optical fibre stub 18 and surrounding ferrule 19 are
typically less than or about 10 mm long. The ferrule 19 is itself
held in place by a ferrule holder 20 which makes a push fit with a
part 21 of the stepped bore 3 and, in addition, is secured in place
by means of adhesive.
[0039] A fibre optic connector 28, part of which is shown in
phantom outline, has a projecting connector ferrule 23, which can
be removably plugged, into an optical port 24 in the form of an
open end of the body 2.
[0040] Fitted about the ceramic ferrule 19 within the body 2 is a
split ferrule 22, which is arranged to centre and lightly clamp the
connector ferrule 23 when the fibre optic connector 28 is connected
to the fibre optic transmission unit.
[0041] The connector ferrule 23 holds a transmission optical fibre
25, one end 26 of which is brought into a co-linear alignment with
an external end 27 of the optical fibre stub 18, so that the
optical radiation which is focussed into the stub 18, is coupled
for onward transmission by the transmission fibre 25.
[0042] The ceramic ferrule 19 within the body 2 has an end 16
facing the laser transmitter module 5 which, together with the end
17 of optical fibre stub 18 is set at a non-normal angle to a
transmission axis 30 of the optical fibres 18,25. As will be
explained in more detail below, the arrangement is such that the
overall coupling efficiency of optical radiation into the optical
fibre stub 18 is reduced, while at the same time coupling
efficiency into the single-mode core 31 of the fibre stub 18 is
made compensatingly higher, and the coupling efficiency into the
cladding 29 is made correspondingly lower. This is done so that the
laser diode 9 can be operated at a higher optical power level for
higher data rate transmission, while at the same time keeping the
light output from the open end 24 of the body 2 from both core 31
and cladding 29 to an eye-safe level when no optical fibre
connector 28 is joined to the optical transmitter unit 1.
[0043] Reference is now made to FIGS. 2 and 3, which show schematic
drawings of a prior art optical transmitter unit 101 similar to
that of FIG. 1, in which components similar to those of the optical
transmitter unit 1 are indicated by reference numerals incremented
by 100. Optical radiation 132 is emitted by the laser diode 109
along an optical axis 133 towards a spherical focusing lens 112.
The optical axis 133 is offset with respect to a centre point 134
of the lens 112 so that the optical axis 135 of optical radiation
136 focussed by the lens 112 is at an angle to the optical axis 133
of the laser diode 109. The main reason for the off-axis focusing
arrangement is so that back reflections 137 from optical components
112,115,118 do not feed back into the laser diode 109. A
consequence of this is that the entry face 117 of the optical fibre
stub 118 is at a non-normal angle to the optical axis 135 of light
focused towards the optical fibre stub 118 from the spherical lens
112.
[0044] In optical diode transmission systems where high data rate
is needed, for example between 1 GHz and 10 GHz, the laser diode
109 is run at higher optical powers in order to achieve faster rise
and fall times in the optical signal. In order to keep the amount
of optical radiation emitted from the fibre stub 118 at an eye-safe
level, the optical radiation 136 projected towards the fibre 118 is
defocused so that this comes to a focus at a beam waist 138 in
advance of the entry face 117 of the optical fibre stub 118.
Optionally, a polarising isolator element 139 may be affixed to the
fibre 118, in which case the entry face 140 of the polarising
element 139 is the effective entry face for the optical fibre stub
118. Such isolators use circular polarisation together with a
quarter-wave plate (not shown) to reduce or stop back-reflections
73 transmitted down the fibre stub 118 reaching the laser diode 9.
However, because such isolators 139 are relatively expensive, they
are not used in applications where such isolation is not
necessary.
[0045] Although defocusing of the focused radiation 136 has the
effect of reducing the optical power 141 launched into the
single-mode core 131 of the optical fibre stub 118, this does have
the effect of launching relatively more optical radiation 142 into
the fibre cladding 129.
[0046] Optical radiation 142 launched into the cladding 129 will
gradually be dissipated by irregular reflections and scattering by
the core 131 and external surface 143 of the cladding. However, a
relatively short fibre stub 118 of the order of about 10 mm may not
sufficiently attenuate unwanted stray optical radiation 142 to an
eye-safe level at the output recess of the optical transmission
unit 101. Although the length of the fibre stub 118 could, in
principle, be increased to reduce the level of stray optical
radiation 142, in practice this would require an optical stub up to
300 mm in length, which is impractically large for commercial
optical transmission units 101, which need to be compact.
[0047] Various solutions have been proposed to this problem, such
as roughening the outer surface 143 of the cladding 129, or
incorporating optical absorbing or blocking structures into the
cladding such as annular grooves filled with optically absorbing
material, or opaque masks on an output face of the cladding at the
external end 27 of the stub. These are additional, inconvenient
process steps in the formation of a laser transmitted unit,
requiring precision etching or masking of the fibre stub 118.
[0048] FIG. 4 shows the orientation of fibre entrance face surfaces
117,140 with respect to the focus axis 135 and fibre transmission
axis 130. When the fibre entrance face 117,140 is at an angle of
between about 6.degree. to 8.degree., then the amount of optical
radiation 141 coupled into the optical fibre core 131 is maximised
when the focus axis 135 is at an angle of 3.degree. to a line
parallel to the fibre transmission axis 130, and at an angle 147 of
between about 9.degree. to 11.degree. to a normal 148 to the fibre
entrance face 117,140.
[0049] FIGS. 5 and 6 show schematically the alignment steps used in
making the optical transmission unit 1 of FIG. 1. The optical
arrangement is initially set up as shown in FIG. 5, in a manner
similar to that described above for FIG. 2, with the exception that
the focused optical radiation 36 is brought to a focus spot 50,
preferably a Gaussian beam waist, on an entry face 17 of the
optical fibre stub 18 in order to couple the maximum amount of
optical radiation 51 into the fibre core 31, and to minimise the
amount of optical radiation 52 launched into the cladding 29. At
this stage, the amount of optical radiation emitted from the open
end 24 of the body 2 will be beyond eye-safe levels at the optical
powers required for high data rate transmission.
[0050] Because the optical fibre stub 18 is securely bonded within
the ceramic ferrule 19, any change to the orientation of the
ferrule 19 will have a corresponding change on the orientation of
the fibre stub 18. The fibre 18 and surrounding ferrule 19 are
cylindrically symmetric, so that a rotation of the ferrule does not
significantly change the orientation of the core 31 at the entry
face 17 with respect to the focus spot 50.
[0051] The ceramic ferrule 19 is then rotated 53 about the fibre
transmission axis 30, in order to change the input coupling angle
47 with respect to the focus axis 35, while at the same time
maintaining the beam waist 50 on the fibre entry face 17 at the
fibre core 31. It may be necessary to realign the focus spot 50
during this procedure to keep this focussed on the entry face 17 of
the fibre core 31. This rotation 53 has the effect of increasing
back-reflected optical radiation 55 from the fibre entry face 17,
so reducing the coupling efficiency of optical radiation 51,52 both
into the fibre core 31 and fibre cladding 29. If the ceramic
ferrule 19 and optical fibre stub 18 are rotated fully by
180.degree. as shown in FIG. 6, then there will be a maximum
reduction in the coupling efficiency of optical radiation 51,52
into the optical fibre stub 18. In general, however, it may not be
necessary to minimise the coupling efficiency to achieve
eye-safety, and so the rotation 53 of the ferrule 19 and optical
fibre stub 18 will, in general, be less than 180.degree.. The main
benefits of the invention will become apparent when the coupling
efficiency has been reduced by at least about 10% from the maximum
as shown in FIG. 5.
[0052] A second embodiment of an optical transmitter unit 201 is
shown schematically in FIGS. 7 and 8. This embodiment is the same
as that illustrated in FIGS. 1, 5 and 6 with the exception that the
optical fibre stub 218 includes an adhered polarising element 39.
The polarising element 39 may be a circular polarising isolator if
this is required, but in the present example is a less expensive
linear polarising element.
[0053] When the ceramic ferrule 19 and optical fibre stub 218 are
initially orientated for maximum optical coupling efficiency as
shown in FIG. 5, the polarising element 39 is initially orientated
with a linear polarising axis 60 parallel to the polarisation
direction 61 of optical radiation 32 emitted by the laser diode 9.
Then, as the ceramic ferrule 19 and optical fibre stub 218 are
rotated 253 about the transmission axis 30 in order to change the
input coupling angle 247 with respect to the focus axis 35, the
efficiency of optical radiation 251,252 coupled into the fibre core
31 and cladding 29 will be synergistically reduced both by the
change in relative orientation 62 between the polarising axis 60
and polarisation direction 61, and also by the reduced coupling
efficiency owing to the non-optimal changes to the input coupling
angle 47,247 for focused optical radiation 36 incident on an entry
face 40 of the polarising element 39.
[0054] Once the desired reduction in coupling efficiency into the
optical fibre stub 218 has been achieved, the orientation between
the ceramic ferrule 19 and body 2 can be set, for example by an
adhesive (not shown).
[0055] The invention provides a number of benefits. First, the
optical power in a fibre core 31 delivered at the output 24 of a
fibre stub 18,218, can be varied according to the needs of the
application, without breaching a maximum allowable total amount of
optical power coupled into the cladding 29 and core 31.
[0056] Laser diodes can be driven to the high optical powers
demanded by many high data rate applications, while limiting the
total optical power emitted by the output of the transmitter unit
to an eye-safe level.
[0057] Since the fibre stub and surrounding ferrule are always
aligned such that the beam waist is positioned in the centre of the
core entrance face, variations in coupled power resulting from
mechanical changes in the assembly, for example those caused by
thermal or ageing effects, or the stub fixing process, are less
pronounced and make the optical performance of the assembly more
stable.
[0058] It is also possible to use a single standard arrangement of
components which can be aligned to achieve a wide range of launch
powers without any change in the dimensions of such components.
This is a significant advantage in comparison with the prior art
approach, which is to set an optical power within a desired range
by changing the separation distance between the input face of the
fibre stub and lens, which can lead to a large variation in the
length of such an assembly.
[0059] When combined with a linear polariser on the fibre stub, the
invention provides a greater range of possible launch powers.
[0060] The invention also requires no apertures or other
light-blocking structures on the fibre cladding.
[0061] The optical transmitter units according to the invention
1,201 described above conform to the US Standard CDRH Class 1 or
the European Standard IEC 825, as regards the emitted radiance from
the unit with no optical transmitter link 28 connected to the
optical port 24. The invention requires no special modifications to
the fibre stub 18 to absorb or block unwanted optical radiation in
the cladding 29. The process steps used in the formation of the
transmitter unit do not require additional processing equipment.
The invention therefore provides a convenient way to achieve higher
laser operating powers, required by high data rate applications,
while at the same time helping to limit the total optical power
emitted by the optical transmitter unit to an eye-safe level.
* * * * *